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<?php
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/*========================================================
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/* Implementation of Douglas-Peuker in PHP.
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/*
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/* Anthony Cartmell
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/* ajcartmell@fonant.com
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/*
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/* This software is provided as-is, with no warranty.
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/* Please use and modify freely for anything you like :)
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/* Version 1.2 - 18 Aug 2009 Fixes problem with line of three points.
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/* Thanks to Craig Stanton http://craig.stanton.net.nz
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/* Version 1.1 - 17 Jan 2007 Fixes nasty bug!
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/*========================================================*/
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class PolylineReducer {
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private $original_points = array();
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private $tolerance;
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private $tolerance_squared;
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public function __construct($geopoints_array) {
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foreach ($geopoints_array as $point) {
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$this->original_points[] = new Vector($point->latitude, $point->longitude);
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}
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/*----- Include first and last points -----*/
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$this->original_points[0]->include = true;
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$this->original_points[count($this->original_points)-1]->include = true;
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}
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/**
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* Returns a list of GeoPoints for the simplest polyline that leaves
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* no original point more than $tolerance away from it.
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*
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* @param float $tolerance
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* @return Geopoint array
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*/
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public function SimplerLine($tolerance = 0.5) {
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$this->tolerance = $tolerance;
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$this->tolerance_squared = $tolerance * $tolerance;
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$this->DouglasPeucker(0, count($this->original_points) - 1);
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foreach ($this->original_points as $point) {
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if ($point->include) {
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$out[] = new GeoPoint($point->x, $point->y);
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}
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}
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return $out;
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}
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/**
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* Douglas-Peuker polyline simplification algorithm. First draws single line
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* from start to end. Then finds largest deviation from this straight line, and if
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* greater than tolerance, includes that point, splitting the original line into
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* two new lines. Repeats recursively for each new line created.
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*
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* @param int $start_vertex_index
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* @param int $end_vertex_index
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*/
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private function DouglasPeucker($start_vertex_index, $end_vertex_index) {
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if ($end_vertex_index <= $start_vertex_index + 1) // there is nothing to simplify
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return;
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// Make line from start to end
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$line = new Line($this->original_points[$start_vertex_index], $this->original_points[$end_vertex_index]);
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// Find largest distance from intermediate points to this line
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$max_dist_to_line_squared = 0;
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for ($index = $start_vertex_index+1; $index < $end_vertex_index; $index++) {
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$dist_to_line_squared = $line->DistanceToPointSquared($this->original_points[$index]);
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if ($dist_to_line_squared>$max_dist_to_line_squared) {
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$max_dist_to_line_squared = $dist_to_line_squared;
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$max_dist_index = $index;
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}
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}
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// Check max distance with tolerance
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if ($max_dist_to_line_squared > $this->tolerance_squared) {// error is worse than the tolerance
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// split the polyline at the farthest vertex from S
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$this->original_points[$max_dist_index]->include = true;
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// recursively simplify the two subpolylines
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$this->DouglasPeucker($start_vertex_index,$max_dist_index);
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$this->DouglasPeucker($max_dist_index,$end_vertex_index);
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}
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// else the approximation is OK, so ignore intermediate vertices
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}
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}
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?>
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